Atrophic rhinitis is a disease which profoundly affects the bone structure of the porcine snout. The etiological agent which is currently considered to be the cause of growth retarding progressive atrophic rhinitis is toxigenic (toxin-producing) strains of P. multocida which colonize the nasal cavity of pigs (Pedersen and Barfod, 1981, (ref. 1), Rutter and Rojas, 1982, (ref. 2), Elling and Pedersen, 1985, (ref. 3), Pedersen et al. 1988 (ref. 4). It has been shown that the nasal mucosa are more easily colonized by P. multocida when the resistance to infection is lower such as when the pigs are concomitantly infected with Bordetella bronchiseptica or when the nasal mucosa are exposed to a mild chemical irritant (cf. Pedersen and Elling, 1984, (ref. 5).
The pathological effects of P. multocida infection may be ascribed to a toxin produced by this bacterium. The toxin which has an apparent molecular weight of 143 kd and an actual molecular weight of 146.5 kd induces bone resorption (osteolysis) of the nasal turbinates and other bone structures in the nasal cavity by stimulating osteoclast activity in porcine turbinate bones, and causes impaired osteoblastic bone formation.
The disease is of major economic importance to pig breeders all over the world, since apart from the pathological effects on the nasal (and occasionally facial) bones noted above, it causes a slower growth rate of the infected pigs and consequently higher production costs. Attempts have therefore been made to reduce the occurrence and the significance of P. multocida infection, for instance by the establishment of SPF (specific pathogen free) pigs via cesarean section, or by antibiotic treatment of infected animals or prophylactic vaccination.
Known vaccines for the immunization of animals, principally pigs, against diseases ascribable to P. multocida infection, especially atrophic rhinitis, comprise killed P. multocida cells optionally combined with killed Bordetella bronchiseptica cells (cf. EP 85 469) and/or an inactivated (usually by heat treatment or addition of formaldehyde) toxin-containing extract of toxigenic P. multocida. Vaccines of the latter type are commercially available from Northern Drugs & Chemicals Ltd., Copenhagen, Denmark, under the trademark Atrinord.RTM., as well as from Intervet International BV, Boxmeer, Holland, under the trademark Nobi-vacART.RTM..
The present inventors contemplate that an improved immunogenic effect relative to the known vaccine preparations may be obtained by using a purified and suitably modified toxin preparation for vaccination purposes either to replace the conventional vaccines or as a constituent thereof.
The purification of P. multocida toxin has previously been described. Thus, Foged et al., 1987, (ref. 6) disclose the purification of the toxin by chromatography and polyacrylamide gel electrophoresis. The purified toxin is used solely for studying its toxic and pathological effects. Kamp et al., 1987, (ref. 7) also disclose the purification of the P. multocida toxin for the purpose of clinical studies. They suggest that the purified toxin may be used as an antigen to raise specific antibodies useful for serological tests. Nakai et al., 1984, (ref. 8) disclose a method of purifying the P. multocida toxin by chromatography and polyacrylamide gel electrophoresis. They further disclose the production of polyclonal antibodies directed against the purified toxin which they use to analyse the purity of the purified toxin. It is suggested that the antibodies may be used to further study the role of the toxin in atrophic rhinitis.
None of these publications suggest the use of a purified toxin as a component of a vaccine for immunizing animals against Pasteurella infection, and this is believed to be a novel concept.